1. Field of the Invention
The present invention relates to a circularity measuring apparatus and, more particularly, an improvement in a protecting mechanism of a detector with a contact type stylus.
2. Description of the Related Art
In order to collect various data about a deviation from circular form such as circularity, concentricity, coaxiality, or the like, of a measured object such as a circular column, a circular cylinder, or the like, the circularity measuring apparatus is well known. In the circularity measuring apparatus, the measured object is loaded on a rotary table, and then the rotary table is turned while detecting a surface position of a measured rotating body with a contact type stylus, or the like, so that data of surface positions of the measured rotating body are collected and thus the circularity is measured/calculated, as disclosed in JP-A-1-259211, JP-A-6-300505, JP-A-2003-302218, JP-A-2004-108787, JP-A-2007-71726, WO2006/008891 and Japanese Patent No. 2701141.
However, when a notch, especially an edged notch, or the like exists on an outer periphery of the measured object, the stylus falls into the notched portion, and the stylus is caught by an edge portion of the notch. When the rotary table continues to turn as it is, the stylus may be broken down.
Therefore, in the prior art, when the stylus comes up to the notched portion, a measuring program is suspended once, and then the stylus is fixed such that this stylus does not fall into the notched portion. Then, the stylus is caused by turning the rotary table to pass through the notched portion, and then the measuring program is started once again.
Similarly, when a measurement of an inner surface of a bore is performed, the breakage of the stylus may also be caused. More particularly, normally a holder for holding the stylus is considerably larger in diameter than the stylus. Therefore, the stylus is positioned to have an inclination such that an outer diameter portion of the holder does not contact the measured object. For this reason, a top end of the stylus is not always positioned on a center line of the holder. Upon inserting the stylus into the bore, the top end of the stylus being held obliquely is caught by an outer periphery of the bore even though the holder is positioned just over the bore, and thus the stylus may also be broken down.
Therefore, in this case, the measuring program is also suspended once upon inserting the stylus into the bore, then the stylus is fixed, then the stylus is inserted into the bore, and then the fixation of the stylus is released.